[PATCH] D123453: [InstCombine] Fold mul nuw+lshr to a single multiplication when the latter is a factor

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 updated this revision to Diff 422122.
bcl5980 added a comment.

rebase


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123453/new/

https://reviews.llvm.org/D123453

Files:
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/shift-logic.ll


Index: llvm/test/Transforms/InstCombine/shift-logic.ll
===================================================================
--- llvm/test/Transforms/InstCombine/shift-logic.ll
+++ llvm/test/Transforms/InstCombine/shift-logic.ll
@@ -254,3 +254,50 @@
   %t27 = ashr exact i32 %t0, 16
   ret i32 %t27
 }
+
+define i64 @lshr_mul(i64 %0) {
+; CHECK-LABEL: @lshr_mul(
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 13
+; CHECK-NEXT:    ret i64 [[TMP2]]
+;
+  %2 = mul nuw i64 %0, 52
+  %3 = lshr i64 %2, 2
+  ret i64 %3
+}
+
+define i64 @lshr_mul_negative_noexact(i64 %0) {
+; CHECK-LABEL: @lshr_mul_negative_noexact(
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 53
+; CHECK-NEXT:    [[TMP3:%.*]] = lshr i64 [[TMP2]], 2
+; CHECK-NEXT:    ret i64 [[TMP3]]
+;
+  %2 = mul nuw i64 %0, 53
+  %3 = lshr i64 %2, 2
+  ret i64 %3
+}
+
+declare void @use(i64)
+
+define i64 @lshr_mul_negative_oneuse(i64 %0) {
+; CHECK-LABEL: @lshr_mul_negative_oneuse(
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 52
+; CHECK-NEXT:    call void @use(i64 [[TMP2]])
+; CHECK-NEXT:    [[TMP3:%.*]] = lshr exact i64 [[TMP2]], 2
+; CHECK-NEXT:    ret i64 [[TMP3]]
+;
+  %2 = mul nuw i64 %0, 52
+  call void @use(i64 %2)
+  %3 = lshr i64 %2, 2
+  ret i64 %3
+}
+
+define i64 @lshr_mul_negative_nonuw(i64 %0) {
+; CHECK-LABEL: @lshr_mul_negative_nonuw(
+; CHECK-NEXT:    [[TMP2:%.*]] = mul i64 [[TMP0:%.*]], 52
+; CHECK-NEXT:    [[TMP3:%.*]] = lshr exact i64 [[TMP2]], 2
+; CHECK-NEXT:    ret i64 [[TMP3]]
+;
+  %2 = mul i64 %0, 52
+  %3 = lshr i64 %2, 2
+  ret i64 %3
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1163,15 +1163,23 @@
       }
     }
 
-    // Look for a "splat" mul pattern - it replicates bits across each half of
-    // a value, so a right shift is just a mask of the low bits:
-    // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
-    // TODO: Generalize to allow more than just half-width shifts?
-    const APInt *MulC;
-    if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
-        BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
+    if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC)))) {
+      // Look for a "splat" mul pattern - it replicates bits across each half of
+      // a value, so a right shift is just a mask of the low bits:
+      // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
+      // TODO: Generalize to allow more than just half-width shifts?
+      if (BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
         MulC->logBase2() == ShAmtC)
-      return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
+        return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
+
+      if (Op0->hasOneUse()) {
+        APInt NewMulC = MulC->lshr(ShAmtC);
+        // if c is divisible by (1 << ShAmtC):
+        // lshr (mul nuw x, c), ShAmtC -> mul nuw x, (c >> ShAmtC)
+        if (MulC->eq(NewMulC.shl(ShAmtC)))
+          return BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
+      }
+    }
 
     // Try to narrow a bswap:
     // (bswap (zext X)) >> C --> zext (bswap X >> C')


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